Synchronous servomotor

ABSTRACT

A synchronous servomotor has a slotted stator core (7) with windings placed in the slots (8) thereof, and a rotor core (5) having magnetic poles formed by permanent magnets (6) attached to the core (5). For the purpose of reducing the reluctance torque, at least certain poles are displaced from the positions which correspond to fully uniform distribution of the poles around the rotor core through a displacement angle which corresponds to a fraction of the slot pitch in the stator core. Mutually, different poles are advantageously displaced from the positions corresponding to fully uniform pole distribution through mutually different angles, although in a manner such that the magnetic forces acting on the rotor core balance one another. The permanent magnets (6) attached to the rotor core ad forming the magnetic poles are suitably of substantially uniform thickness and of arcuate configuration havng a curvature of radius which is smaller than the radial distance of the permanent magnets from the rotary axis of the rotor core.

The present invention relates to a synchronous servomotor of the kindwhich comprises a stator with a stator core provided with slots and awinding placed therein, and a rotor with a rotor core and field poles inthe form of permanent magnets attached to the rotor core and located inthe air gap between the stator core and the rotor core.

Synchronous servomotors of this kind give rise to a problem generallyreferred to as reluctance torque or reluctance cogging, which is causedby magnetic interaction between the permanent magnetic poles on the onehand and the slots and interlying teeth in the stator core on the other.This magnetic interaction, which is completely independent of thecurrent flowing in the stator winding and is therefore present even whenno current passes therethrough, gives rise to a varying or penduloustorque on the rotor when the rotor is rotated in the stator, this torquevariation being superposed on the driving torque generated by thecurrent in the stator winding. In actual fact, when no current flowsthrough the stator winding, the rotor exhibits a number of preferencepositions which the rotor strives to take, this number being dependenton the number of poles of the motor. This phenomenon causes the motor torun unevenly and results in deviations from desired motor speeds, theabsolute magnitude of the effect on the speed of the motor beinginversely proportional to motor speed and therewith being particularlynoticeable at low speeds. Consequently, the reluctance torque orreluctance cogging constitutes a particularly serious problem inservomotors which must be capable of being driven very precisely with anextremely wide range of speeds, and also at very low speeds.

Endeavours have been made to overcome this problem, by using a smoothstator core without winding slots and by placing the stator winding onthe smooth inner surface of the stator core, the winding being woundwith the aid of a special winding technique. With this solution,however, the stator winding takes up a substantial part of the air gapbetween the stator core and the rotor core, which means that the air gapmust be made larger, resulting in poor use of machine dimensions.Another known method of reducing the reluctance torque is to design themotor with skewed slots and/or field poles. Although both of thesesolutions provide an acceptable reduction of the reluctance torque theyare encumbered with other drawbacks. For example, when using permanentmagnets the use of skewed poles is mechanically difficult and relativelyexpensive to put into effect. The use of skewed slots for the statorwinding also gives rise to certain difficulties in conjunction with themanufacture of the stator winding, particularly since it is desirablethat this can be done mechanically.

Consequently, an object of the present invention is to provide asynchronous servomotor of the kind described in the introduction inwhich the reluctance torque or reluctance cogging is greatly reduced ina simpler and, from the aspect of manufacturer, a far less expensivemanner than has hitherto been possible.

The characterizing features of the servomotor according to the inventionare set forth in the following claims.

The invention will now be described in more detail with reference to theaccompanying drawings, in which:

FIGS. 1-6 are schematic diagrams illustrating the basic principle of theinvention and advantageous embodiments thereof;

FIG. 7 illustrates schematically an advantageous embodiment of thepermanent magnet poles of a servomotor according to the invention;

FIG. 8 is a schematic axial sectional view of a 4-pole servomotorconstructed in accordance with the invention;

FIG. 9 is a schematic sectional view taken on the line IX--IX in FIG. 8.

The fundamental concept of the invention resides in the displacement ofcertain poles relative to positions which correspond to uniform poledistribution by an angle corresponding to a fraction of the pitch of thestator core slots instead of distributing the poles of the motoruniformly around the motor periphery in the conventional manner. Atleast some of the rotor pole pitches will thus deviate from the value360°/p, where p is the number of motor poles. Because at least some ofthe motor poles are displaced or offset from the positions correspondingto fully uniform distribution of the poles around the rotor, differentpoles will occupy different positions in relation to the slots and teethof the stator core thereby, when seen in total, equalising the magneticinteraction between the magnetic poles and the slots and interlyingteeth in the stator core, and reducing the afore-discussed torquevariation, the so-called reluctance cogging or reluctance torque, uponrotation of the rotor in the stator. In this respect the best result isobtained when,if possible, the various field poles are offset ordisplaced by angular values of mutually different magnitudes from thepositions corresponding to a totally uniform pole distribution. Theprinciple on which the invention is based can be illustrated with theaid of FIG. 1, which illustrates schematically the distribution orpositioning of the poles of a 4-pole motor constructed in accordancewith the principle on which the invention is based, in which figure fullline arrows show the directions of the direct axes of the poles, whereasthe broken lines illustrate corresponding directions in a conventionalmotor in which the poles are uniformly positioned. As will be seen fromFIG. 1, in the case of a 4-pole motor constructed in accordance with thefundamental principle of the invention, three of the poles are displacedfrom these positions which correspond to uniform pole distribution, thepoles being displaced to mutually different extents, and advantageouslyto extents corresponding respectively to t/4, t/2, and 3t/4 where t isthe slot pitch in the stator core. The magnitude of pole adjustment canthus be expressed as k·t/p where t is the slot pitch of the stator core,p is the number of poles of the motor, and k is an integer which issmaller than p and of differing magnitude in respect of the mutuallydifferent offset positions of the poles. In theory this provides thebest possible result with respect to reduction of the reluctance torque.

It will be seen, however, that in the case of a motor arrangement of thekind illustrated in FIG. 1, the magnetic forces acting on the rotor arenot balanced, resulting in magnetic lateral pull on the rotor, which isunacceptable in the majority of cases.

This drawback can be avoided, however, by displacing the poles in themanner illustrated in FIGS. 2, 3 and 4 with regard to a 4-pole, a6-pole, and an 8-pole motor respectively. In this embodiment of theinvention two diametrically opposed poles are always displaced from thepositions corresponding to complete uniformity in pole distribution tomutually the same extent and in mutually the same direction, thus in thecase of a 4-pole motor one pair of mutually diametrically opposed poleswill be displaced in relation to the said positions, as illustrated inFIG. 2, whereas in the case of motor having 6 or 8 poles two or threepairs of poles, respectively, are displaced relative to said positions,as shown in respective FIGS. 3 and 4.

The extent to which the diametrically opposed poles of each pair aredisplaced advantageously corresponds to the value of k·2t/p, where t isthe pitch of the stator slots, p is the motor pole number and k is aninteger which is smaller than p/2 and which is different for thedifferent pole pairs being displacement.

Although the afore-mentioned pole positions illustrated by way ofexamples in FIGS. 2-4 afford theoretically the best result with regardto the desired reduction in the reluctance torque of the motor, a fullysatisfactory result can, in many cases, be achieved in a somewhatsimpler manner with respect to motors which incorporate four or morepoles. Thus, as illustrated in FIGS. 5 and 6, in the case of arespective 6-pole and 8-pole motor, each alternate pole is displacedfrom the position which corresponds to a fully uniform poledistribution. In this case, all of the displaced poles are offset atmutually equal distances from the aforesaid positions, preferablythrough an angle corresponding to t/2, and in mutually the samedirection. This positioning of the poles will also ensure that themagnetic forces acting on the rotor are balanced, so as to avoidmagnetic lateral pull on the rotor.

It can also be mentioned that the pole displacement effected whenpractising the invention does not unduly influence the winding factor ofthe motor, and hence does not lower the efficiency of the motor to anyappreciable extent.

Pole displacement in accordance with the present invention can beadvantageously combined with the aforementioned known concept of skewedstator slots in a synchronous servomotor. These two features co-act in ahighly advantageous manner to provide the desired reduction in thereluctance torque of the motor. When both of these features or measuresare incorporated the motor reluctance torque is completely eliminatedfor all practical purposes.

Similar to skewed stator slots a pole displacement according to theinvention results in a decrease in the slot ripple in the inducedvoltage of the motor. This is of minor interest in the case ofsynchronised servomotors, since the slot ripple of the induced voltageis of subordinate significance in this type of machine.

A much more significant feature with respect to the present invention isthat the displacement of the rotor poles formed by permanent magnets canbe effected very simply, without adding to the costs of manufacturingthe motor.

A further improvement can be achieved in synchronous servomotors of thekind to which the invention relates with regard to the desired reductionin reluctance cogging and the torque pulsation caused thereby, by givingthe permanent magnets forming the rotor poles a particular form. FIG. 7illustrates schematically part of the rotor core 5 of a synchronousservomotor having mounted thereon a permanent magnet 6 which forms afield pole, the rotor core part being straightened out from its normallycylindrical configuration for illustration purposes. The figure alsoillustrates part of the stator core 7 and its winding slots 8 locatedopposite the illustrated part of the rotor core 5. As illustrated inFIG. 7, according to one advantageous further development of theinvention the permanent magnet 6 forming a rotor pole is of substantialyuniform thickness and of arcuate configuration with a radius ofcurvature which is smaller than the radial distance of the permanentmagnet 6 from the axis of rotation of the rotor core 5. Consequently theair gap between the rotor core 5 and the stator core 7 is much greaterat the side edges of the permanent magnet 6, and therewith at the sideedges of the field pole and in the pole gaps than at the centre regionof the field pole. This also contributes towards a reduction in thereluctance cogging of the motor. It will be noted in this connectionthat because of the low permeability of modern permanent-magnetmaterial, the permanent magnet 6 from the aspect of magnetic flux can begeneraly equated with an air gap. This particular configuration of thepermanent magnet 6 forming the rotor poles of the machine also affordsthe additional highly beneficial advange that the quadrature flux in themotor becomes much smaller as a result of the increased air gap in thepole gaps. Consequently a larger maximum torque can be developed by themotor without requiring additional expensive permanent-magnet material.At the same time there is also obtained a straighter torque-currentcharacteristic of the motor.

FIGS. 8 and 9 illustrate schematically and by way of example the rotorof a 4-pole synchronous servomotor constructed in accordance with theinvention. The rotor has a shaft 1 on which there is mounted a laminatedrotor core 2. The field poles of the rotor 4 are formed by permanentmagnets 3 which are held in position by means of holders 4, made forexample of aluminum or some other suitable non-magnetic material.Alternatively, the permanet magnets may be attached to the rotor core 2with the aid of a suitable adhesive and/or by taping.

The permanent magnets 3 are positioned in the aforedescribed mannerillustrated in FIG. 2 and have the configuration illustrated in FIG. 7and described above, having a radius curvature which is substantiallysmaller than the circumferential radius of the rotor core 2, i.e.smaller than the radial distance of the permanent magnets from therotational axis of the rotor.

I claim:
 1. A synchronous servomotor comprisinga stator with a slottedstator core and stator windings located in the slots in said core, and arotor with a rotor core and field poles comprising permanent magnetsmounted on the rotor core and located in an air gap between the statorcore and the rotor core, wherein respective ones of said poles aredisplaced from positions corresponding to fully uniform distribution ofthe poles around the rotor through an angle which corresponds to afraction of the slot pitch in the stator core, such that the pole pitchbetween mutually adjacent poles differs from that of 360°/p, where p isthe number of said poles.
 2. A synchronous servomotor according to claim1, wherein mutually different ones of said permanent magnet field polesare displaced through mutually different angles from said positionscorresponding to a fully uniform peripheral spacing of the poles aroundthe rotor.
 3. A synchronous servomotor according to claims 1 or 2,wherein the displaced poles are displaced through angles correspondingto k·t/p, where t is the slot pitch of the stator core and k is aninteger smaller than p.
 4. The servomotor of claim 1, wherein the numberof said field poles is even, and, with the exception of one pair of saidmutually diametrically opposed permanent magnet field poles, each of theremaining pairs of diametrically opposed permanent magnet field poles isdisplaced from a position corresponding to a fully uniform peripheralspacing of the field poles around through an angle corresponding to afraction of the slot pitch in the stator core.
 5. The servomotor ofclaim 4, wherein said angle of displacement has a value of k·2t/p, wheret is the slot pitch of the slot pitch of the stator core and k is aninteger which is smaller than p/2 and which is mutually different fordifferent ones of said displaced pairs of diametrically opposedpermanent magnet field poles.
 6. The servomotor of claim 1, wherein eachalternate one of said permanent magnet field poles is displaced from aposition corresponding to a fully uniform peripheral spacing of thepermanent magnet field poles around said rotor through an anglecorresponding to a fraction of the slot pitch in the stator core, theangle of displacement and the direction thereof being the same for allsaid alternate permanent magent field poles.
 7. The servomotor of claim6, wherein said displacement angle has a value of t/2, where t is theslot pitch in the stator core.
 8. The servomotor of claim 1, wherein theslots in the rotor core are skewed.
 9. The servomotor of claim 1,wherein each of the permanent magnets forming said field poles of saidrotor has a substantially uniform radial thickness and an arcutateconfiguration with a radius of curvature which is smaller than theradial distance of the permanent magnet from the rotor axis of saidrotor core, such that the radial distance between the rotor core and thestator core is substantially greater at the side edges of the permanentmagnets and in the pole gaps than at the center of the permanentmagnets.